This invention relates to circuits for recovering the carrier signal on which a digital signal has been modulated, and more particularly to circuits which can cope with rapid phase changes in the carrier signal caused by multipath propagation of radio frequency signals.
Radio frequency carrier signals, which are modulated by digital information signals for transmission to digital portable radio receivers, are subject to rapid phase changes due to the multipath propagation environment in which these systems operate. The multipath environment is created by signal reflections at walls and other interfering objects that cause these rapid phase changes as the portable set is moved therethrough. In particular, these phase changes consist of approximately 180 degree transitions which occur approximately every one-half wavelength of motion through the environment and occur at local amplitude minima in the random amplitude distribution.
In order to detect the modulating digital signal properly, a carrier recovery circuit recovers the frequency and phase of the carrier signal from which the digital modulating signal has been stripped. Conventionally, a phase locked loop (PLL) in carrier recovery circuits maintains a frequency and phase lock between the received signal and the recovered carrier. In order, however, to track rapid 180 degree step-like phase changes, the phase locked loop closed loop bandwidth would be required to be many times the signal variation rate (Doppler spectrum width) caused by motion through the environment since step changes in signal phase require impulsive changes in the voltage controlled oscillator (VCO) within the PLL. Therefore, rapid phase changes require extremely large and sharp VCO frequency changes, i.e., the PLL closed loop bandwidth must be wide. Disadvantageously, such a wide PLL closed loop bandwidth results in noisy carrier recovery during the periods of low signal level which is when these large phase changes occur. Furthermore, during these periods of low signal level, the PLL may loose lock as it attempts to track a rapid phase change by changing the VCO frequency too far from the average signal frequency. As a result, phase locked loop reacquisition may be delayed excessively and may not even occur when the signal amplitude reaches a normal level.